Ministers and officials were accused of trying to keep details of the troubled Universal Credit (UC) project secret by a powerful committee of MPs.
The cross-party Public Accounts Committee said it was concerned that the Major Projects Authority (MPA) awarded the flagship benefit reform a "reset" rating in an attempt to prevent scrutiny.
The decision to reset UC meant it was not given a rating by the MPA on its five-tier traffic light system, running from green to red, in this year's annual report.
The MPA said the decision to award the reset rating was ultimately made by ministers in Work and Pensions Secretary Iain Duncan Smith's department.
The committee raised concerns about the lack of information released by the MPA, particularly over UC which will not have a normal "delivery confidence assessment" traffic light rating until next May.
The MPs said: "We are particularly concerned that the decision to award a 'reset' rating to the Universal Credit project was an attempt to keep information secret and prevent scrutiny."
The report added: "The 'reset' category was introduced for the 2013-14 report and was only applied to this one project.
"The MPA confirmed that the decision to give Universal Credit a reset rating was ultimately made by ministers. However, the MPA also told us that the decision to give this rating 'was a good thing for the project' as it allowed the project team a 'bit of breathing space to get themselves back on the front foot'.
"The MPA c onfirmed that Universal Credit would still be subject to its assurance reviews and that, in line with the transparency policy, a normal delivery confidence assessment would be published in May 2015.
"This is a long time to wait for an update on a project as important as Universal Credit."
The committee called for the MPA, which was established to improve project delivery by Whitehall departments, to be given more powers and for it to explicitly consider whether projects are likely to be good value for money.
The MPs said the MPA's chief executive should have a formal mechanism available to set out his position if a project proceeds contrary to the authority's advice to cancel or change.
There should also be a formal and transparent process to record where ministers or officials reject the MPA's recommendations.
The committee said the Treasury should be given overall responsibility for overseeing the £488 billion portfolio of major projects, including programmes such as HS2 and the new aircraft carriers, rather than treating them as individual schemes.
It also recommended the MPA should focus on departments such as health and defence, where there were a number of risky projects.
Margaret Hodge, the committee's Labour chairwoman, said: " We support the work of the Major Projects Authority and welcome the progress it has made so far, but without stronger powers it is unlikely to achieve its aim of a systemic improvement in project delivery across government.
"The projects in the MPA's portfolio represent a huge and rising cost to the taxpayer. In May 2014, the total cost over the lifetime of these projects was estimated at £488 billion, an increase of some £134 billion on the previous year.
"The portfolio includes high-profile and extremely challenging projects such as Universal Credit and the Queen Elizabeth aircraft carrier programme.
"However, the MPA only has informal influence over departments. It supports the Treasury in approval and funding decisions but there is no obligation on the Treasury to follow its recommendations. It has no powers if a department decides to proceed with a project against MPA advice.
"It needs to have stronger, more formal mechanisms for driving change - and there should be transparency where ministers or officials have rejected its recommendations.
"We constantly encounter projects that suffer from a lack of robust planning. The MPA needs to focus its efforts more on the early stages of a project, working with departments to ensure that they have devoted sufficient attention to the concept, design and business case for projects before seeking approval.
"It could also improve its impact by prioritising its work more effectively, focusing on departments with current high-risk projects - the Department of Health has two red and nine amber-red rated projects, while the Ministry of Defence has one red and six ambers, for example."
A Department for Work and Pensions spokesman said: "This is a historical rating that reflects the decision to reset the programme in 2013. Now this reset has taken place, future Major Projects Authority reports will give a full delivery assessment of Universal Credit.
"In terms of transparency, Universal Credit has had the most reviews and the highest number of reports written about it, out of the 44 major projects.
"The reality is that Universal Credit is already making work pay as we roll it out in a careful and controlled way. In fact it is already transforming people's lives and is available in a growing number of areas, including 39 Jobcentres, and we have started taking claims from couples too. By the end of this year, around one in eight job centres will offer Universal Credit."
A Cabinet Office spokesman said: "The committee's comments are a welcome endorsement of how the MPA is going from strength to strength. As the committee chair notes, we are developing leaders, increasing transparency around major projects and ensuring projects are set up to succeed from the outset to get the best value for money for the taxpayer.
"As part of our long-term economic plan, the MPA has helped departments save the taxpayer over £2.4 billion last year against a 2009/10 baseline. We are looking forward to building on this success and we will respond to the report's recommendations shortly."